1. Field of the Invention
The present invention relates to solar photolysis of water to produce pure hydrogen and to various methods and apparatus for conducting such a process.
2. Description of the Prior Art
Self-sufficiency in energy is a stated national goal.
Most of the proposed means to achieve this goal are either environmentally unacceptable or are not feasible, especially those not depending on fossil fuel sources. Of the available alternatives, solar energy is the most abundant, inexhaustable single resource available. However, capturing and utilizing solar energy is not simple. Methods are being sought to convert solar energy to a concentrated, storable form of energy. A known method, photosynthesis, converts somewhat less than 1% of the sun's energy at the earth's surface to a solid fuel, i.e., plant materials, which when accumulated and transformed over geologic ages yielded fossil fuels. Current rates of use of these fossil fuels, and the particular geographic distribution and political control of major petroleum resources pose problems for nations that are net petroleum consumers. An alternate method yielding a simpler fuel, at a higher conversion, has long been desired.
Production of hydrogen by the solar photolysis of water would be an extremely desirable fuel, since it would be prepared in high purity, and the combustion product of hydrogen is water which is totally environmentally acceptable. However, it is widely believed that solar photolysis of water is not feasible, especially at quantum efficiency exceeding 1%. Douglas and Yost noted twenty four years ago in J. Chem. Phys. 17, 1345 (1949) and J. Chem. Phys. 18, 1687 (1950) that hydrogen was produced during photolysis of europium (II) solutions. Yields of hydrogen were not measured since their main interest was in europium oxidation. This sole reaction would not lead to a feasible process for photolyzing water since the europium ion would be continuously exhausted by stoichiometric reaction with water, therefore the process would be unduly expensive since the amount of hydrogen generated would not economically justify the cost of the europium reagent.
A cyclic photo-redox process having water and sunlight as reactants and hydrogen and oxygen and products has been disclosed in a patent application Ser. No. 658,132 filed Feb. 13, 1976, now issued on Aug. 30, 1977 as U.S. Pat. No. 4,045,315. That process utilized a soluble divalent europium photo-oxidation reagent in the hydrogen generation cycle and a complex series of steps in the dark in which a water-stable manganese oxychloride is utilized to regenerate the spent trivalent europium photo-oxidation reagent. The complexity renders the process less economic in the large scale harvesting of solar energy and the dark reaction requires use of dark panels or waiting for sunset to conduct the regeneration cycle of the process.